As Colin said, all verticals work better the higher they are which is just 
about the way it is with any antenna depending upon what you are trying to 
do.  The reason this is, for a multiple band vertical, is that the lower 
bands, such as 10 and 12 meters, are lower pieces of the vertical tubing. 
Thus, the higher off ground, the fewer radiation blockages by fences and 
houses and trees, kids, dogs, cats, chickens, cows, if mounted in a grazing 
pasture,  etc.

Phil.
K0NX